Thermal ink-jet printers offer a low cost, high quality, and comparatively noise-free option to other types of printers commonly used with computers. Such printers employ a resistor element in a chamber provided with an egress for ink to enter from a plenum. The plenum is connected to a reservoir for storing the ink. A plurality of such resistor elements are arranged in a particular pattern, called a primitive, in a printhead. Each resistor element is associated with a nozzle in a nozzle plate, through which ink is expelled toward a print medium. The entire assembly of printhead and reservoir comprise an ink-jet pen.
In operation, each resistor element is connected via a conductive trace to microprocessor, where current-carrying signals cause one or more selected elements to heat up. The heating creates a bubble of ink in the chamber, which is expelled through the nozzle toward the print medium. In this way, firing of a plurality of such resistor elements in a particular order in a given primitive forms alphanumeric characters, performs area-fill, and provides other print capabilities on the medium.
Recent innovations in thermal ink-jet printing have been directed to providing the capability of color printing. Typically, yellow, magenta, and cyan inks are employed. These three primary colors, when mixed in various combinations, provide a full color palette. Black printing is provided either with a separate black ink or by composite printing of yellow, magenta, and cyan inks.
During printing, a color may require more than one drop of ink at a particular pixel location, either of the same color (to control hue) or of a different color (to provide a specific color). Such multiple drops of ink at one location can provide a heavy dye load on the print medium, or paper.
Heavy dye loads of various colored inks on paper can lead to bleed and reduction of waterfastness. Bleed, as used herein, is the invasion of one color into another color on paper, which is a surface phenomenon. This is in contradistinction to uses of the term in the prior art, which tend to define "bleed" in the context of ink of a single color following the fibers of the paper; this is a sub-surface phenomenon.
Bleed control has been achieved by addition of one or more surfactants; see, e.g., U.S. Pat. No. 5,116,409, issued May 26, 1992, to John Moffatt and assigned to the same assignee as the present application. Alternatively, fast penetrating solvents have been employed to improve bleed resistance; see, e.g., U.S. Pat. No. 5,196,056, issued Mar. 23, 1993, to Keshava Prasad and assigned to the same assignee as the present application.
A need remains for ink compositions for use in ink-jet printing, particularly thermal ink-jet printing, which do not evidence bleed, as defined herein, and yet which possess relatively long shelf life and other desirable properties of such inks.